Method for operating a lighting system and suitable lighting system therefor

ABSTRACT

The invention pertains to a method for operating a lighting system with anncoherently-emitting radiation source, in particular a discharge lamp (14) that emits UV, IR or visible-range radiation, by means of dielectrically inhibited discharge, and to a lighting system suitable therefor. The electrodes (16-20), which are arranged side by side and separated from each other and the interior of the discharge vessel (15) by dielectric material (21), are alternatingly connected to the two poles (23, 24) of a voltage source (27). In operation, the voltage source (27) supplies a series of voltage pulses separated by quiescent periods. According to the invention, this produces inside the discharge vessel (15) a spatial discharge (26) which in the regions between electrodes of different polarity (16, 17; 17, 18; 18, 19; 19, 20) is at a distance from the surface of the inside wall of the discharge vessel (15). Substantial advantages are less stress on the wall of the discharge vessel and greater efficiency in generating radiation.

TECHNICAL FIELD

The invention concerns a method for operating a lighting system with anincoherently emitting radiation source, particularly a discharge lamp,by means of dielectrically impeded discharge in accordance with thepreamble of claim 1. The invention also concerns a lighting systemsuited for the said method of operation in accordance with the preambleof claim 12.

Incoherently emitting radiation sources are understood to be UV(Ultraviolet) and IR (Infrared) radiators as well as discharge lamps, inparticular, those which radiate visible light.

INDUSTRIAL USES

These types of radiation sources are suited, according to the spectrumof the emitted radiation, for general purpose and auxiliary lighting,for example, house and office lighting; for background lighting fordisplays, for example, LCD's (Liquid Crystal Displays); for automotiveand signal lighting; for UV irradiation, for example, degermination orphotolytics; and for IR irradiation, for example, in the drying ofvarnishes.

PRIOR ART

A method for operating an incoherently emitting radiation source,particularly a discharge lamp, by means of dielectrically impededdischarge was revealed in WO 94/23442. This operating method requires asequence of voltage pulses, whereby the individual voltage pulses areseparated from one another by idle times. The advantage of this pulsedoperation method is a high efficiency in the generation of radiation.

EP 0 363 832 describes a UV high-power radiator with electrodesconnected pairwise to the two poles of a high-voltage source. Theelectrodes are separated from one another and from the discharge chamberof the radiator by dielectric material. Such electrodes are hereinafterreferred to as "dielectric electrodes". Also, the electrodes arearranged adjacent to one another in a way that allows flattish dischargeconfigurations with relatively flat discharge chambers. An alternatingvoltage in the magnitude of several 100 V to 20,000 V with a frequencywithin the range of industrial alternating current of up to a few kHz isapplied to the dielectric electrodes so that an electrical creepingdischarge forms essentially only in the region of the dielectricsurface.

The primary disadvantage in this is that the creeping discharges stressthe surface thermally and, therefore, cooling channels for thedissipation of heat from the dielectric are proposed. The efficiency ofthe generation of radiation, particularly in the UV and VUV (VacuumUltraviolet) range, is limited by the unavoidable, substantial heatgeneration of this discharge type. Additionally, a creeping dischargecauses chemical processes on the surface and shortens the life of theradiator.

PRESENTATION OF THE INVENTION

The object of the invention is to avoid these disadvantages and tospecify a method for the operation of a lighting system, which isdistinguished both by a flat discharge chamber and an efficientgeneration of radiation.

This object is achieved according to the invention by the characterizingfeatures of claim 1. Further advantageous features are explained in thesubclaims.

A further object of the invention is to specify a lighting system whichis suited for the aforementioned method of operation. This object isachieved according to the invention by the characterizing features ofclaim 12.

The basic idea of the invention is to generate with adjacent dielectricelectrodes a spatial discharge in the interior of the discharge chamber,which has a spacing from the surface of the interior wall of thedischarge chamber in the regions between electrodes of oppositepolarity. While in the prior art a multitude of creeping dischargesalong the surface of the dielectric serve to generate UV radiation, theinvention suggests the use of a discharge which detaches itself from thedielectric surface and is spatially extended inside the dischargechamber.

The advantages achieved by this are a higher efficiency in thegeneration of UV and/or VUV Lvacuum Ultraviolet) radiation and,therefore, a reduced generation of heat. In contrast to the prior art,no cooling liquid is required for the dissipation of heat. Additionally,the discharge type according to the invention causes thermal andchemical stresses to the wall that are substantially lower than those insurface creeping discharges. Consequently, the life of the dischargechamber is extended. Moreover, in comparison to the prior art, a morehomogenous, flattish, spatially diffuse luminance distribution can berealized according to the invention between the electrodes. The latter,in contrast to the channel-shaped creeping discharges, offerssubstantial advantages in optical image-forming lighting and/orirradiation uses, for example, photolithographic applications wherediffuse luminance distributions substantially increase the efficiency ofthe process. In this respect, luminous patterns such as those producedby the conventional, channel-shaped luminous structures are not desired.

The method according to the invention provides that the adjacentdielectric electrodes are connected to a voltage source which provides asequence of voltage pulses. The individual voltage pulses are separatedfrom each other by pauses. Surprisingly, it was found that by thisprocedure, not only is a radiation of high efficiency generated, butthat unexpectedly, a spatial discharge is generated in the interior ofthe discharge chamber which shows a spacing from the surface of theinner wall of the discharge chamber in the regions between electrodes ofdifferent polarity.

Starting from a repeating voltage pulse, pulse width and pause durationare chosen so that there results the spatial discharge which partiallydetaches itself from the dielectric surface according to the invention.Typical pulse widths and pause durations are in the range between 0.1 μsand 5 μs and 5 μs and 100 μs respectively, corresponding to a pulserepetition frequency in the range between 200 kHz and 10 kHz.

The optimal values for the pulse width and the pause duration depend inthe individual case on the actual discharge configuration, that is tosay, on the type and pressure of the gas filling as well as theelectrode configuration. The electrode configuration is determined bythe type and thickness of the dielectric, the area and shape of theelectrodes, as well as the electrode spacing. Corresponding to thedischarge configuration, the voltage signal to be applied should bechosen so that it generates a discharge which detaches itself from thedielectric surface and that has the maximum radiant efficacy at adesired electric power density. In principle the sequences of voltagepulses disclosed in WO 94/23442 are also suited for this. The height ofthe voltage pulses is typically between about 100 V and 10 kV. The shapeof the current pulses is determined by the shape of the voltage pulseand by the discharge configuration.

Two or more longish electrodes of electrically conductive material, forexample metallic wires or strips or also narrow metal coatings appliedto, for example, vapor-deposited on, the exterior of the chamber wallare suited for the electrode configuration. It is preferred that theelectrodes are arranged parallel to and equidistant from one another.This is important in order to ensure the same conditions for alldischarges between the respectively neighboring electrodes. A wide-area,homogenous illumination is thereby assured. Additionally, in this manneran optimal radiant efficiency is achieved by a suitable sequence ofpulses. The lateral dimensions--that is to say, the diameters of thewires or the widths of the strips--can be different from anode tocathode.

The operating method according to the invention is suited for a varietyof possible discharge chamber geometries, in particular for all of thosethat are specified in EP 0 363 832 A1. It is also of no consequencewhether the discharge chamber contains a gas filling and is sealed ingas-tight manner as, for example, in discharge lamps, or whether thedischarge chamber is open on both sides and has a gas or a gas mixtureflowing through it, as for example, in photolytic reactors. It is onlyrequired for the method of operation that the dielectric electrodes arearranged next to one another. Next to one another in this case meansthat neighboring electrodes of different polarity are both located onone side of the discharge zone.

The electrodes can be arranged in a common plane, for example on theexterior surface of a wall of the discharge chamber--possiblyadditionally covered by a dielectric protective layer--or alternatively,directly imbedded in the chamber wall. Additionally, it is possible toarrange the electrodes in different and preferably mutually parallelplanes on one side of the discharge zone. For example, depending onpolarity, the successive electrodes of alternating polarity are arrangedin one of two mutually offset planes, as published, for example, inDE4036122A1.

In plane discharge chambers the base or top surface advantageouslyserves as the wall on which the electrodes are arranged. Plane dischargearrangements are particularly suited for large area, plane illumination,for example, as back lighting for indicator panels or LCD screens, aswell as for irradiation uses such as in photolithography or the curingof varnishes.

Besides plane arrangements, curved discharge chambers, for example,tubular ones, are also suited. Tubular arrangements with both sides openand through which gas or a gas mixture flows are particularly suited asphotolytic reactors. In its simplest design a tubular arrangement isformed by a dielectric tube, for example with a circular cross-section.The electrodes in this case are arranged at least on or in a part of theexterior or of the wall of the tube. The discharge forms in the interiorof the tube during operation. In a variant, the interior wall of thetube is coated in the region of the elecctrodes with a dielectric layerwhich serves as an optical reflector.

A further development on the tubular arrangement consists of twoconcentric tubes of different diameters and electrodes arranged on or inthe interior wall of the tube with the smaller diameter. The dischargeforms in the space between the two tubes during operation.

The interior wall of the discharge chamber can be coated with a phosphorcoating which converts the UV and VUV radiation of the discharge intolight. A variant with a phosphor coating that emits a white light isparticularly suited for general lighting purposes.

The selection of the ionizable filling and, when applicable, thephosphor coating is determined by the aim of application. Inert gases,for example, neon, argon, krypton and xenon, as well as mixtures ofinert gases are particularly suited. However, other filling substancescan be used, for example, all of those which are commonly used in thegeneration of light, particularly mercury (Hg) mixtures and inertgas/mercury mixtures as well as rare earths and their halides.

The lighting system is completed by a voltage source, the output polesof which are connected to the electrodes of the discharge chamber andwhich delivers the aforementioned sequence of voltage pulses duringoperation.

DESCRIPTION OF THE ILLUSTRATIONS

The invention is explained in more detail below by a few embodiments inwhich

FIG. 1a shows the cross-section of a discharge arrangement having twodielectric electrodes arranged next to one another,

FIG. 1b shows the longitudinal section of the discharge arrangement inFIG. 1a,

FIG. 2 shows the end view of the discharge arrangement from FIG. 1a inoperation according to the invention,

FIG. 3 shows a detail from the temporal characteristic of current l(t)and voltage U(t) as measured at the electrodes during operation inaccordance with FIG. 2,

FIG. 4 is as FIG. 2, but with altered electrode geometry,

FIG. 5 shows a detail from the temporal characteristic of current I(t)and voltage U(t) as measured at the electrodes during operation inaccordance with FIG. 4,

FIG. 6a shows the cross-section of a lighting system suited for theoperation according to the invention,

FIG. 6b shows the top plan view of the lighting system in FIG. 6a.

FIGS. 1a and 1b show a schematic representation of the cross andlongitudinal sections of a discharge arrangement 1. In order to be ableto better explain the core of the invention, and to further clarity, therepresentation is deliberately reduced to what is essential. Thedischarge arrangement 1 consists of a cuboid, transparent dischargechamber 2 and two parallel, strip-shaped electrodes 3, 4 which arearranged on the exterior wall of the discharge chamber 2. It may bepointed out once again at this point that similar discharge arrangementswith more than two dielectric electrodes of opposite polarity arrangednext to one another are, of course, equally suited for the operatingmethod according to the invention. The discharge chamber 2 is made ofglass. It consists of a cover 5 and a base 6 which are bothtrough-shaped and are positioned in mirrored fashion across from oneanother; two side walls 7, 8 which define the longitudinal axis of thedischarge chamber 2 and two end walls 9, 10. The interior of thedischarge chamber 2 is filled with xenon at a filling pressure ofapproximately 8 kPa. The two electrodes 3, 4 are made from aluminumfoil. They are adhered to the exterior of the cover 5 centrally and inparallel. The cover 5 is made of glass of 1 mm thickness and functionsadditionally as a dielectric layer between the two electrodes and thedischarge 11--which is depicted here only in a rough schematicillustration--which forms in the interior of the discharge chamber 2during operation. According to the invention, the discharge 11 isseparated from the interior wall of the cover 5 in the region betweenthe two electrodes 3, 4 by a dark zone 12 (in longitudinal section, FIG.1b, not discernible). That is, the discharge 11 has a spacing from thesurface of the interior wall in the aforementioned region.

FIGS. 2 and 4 show photographs of the discharge arrangements from FIGS.1a and 1b. The corresponding reference numbers used above are again usedto explain the photographs. The two photos were both taken with a viewtowards the end wall 9 in the direction of the longitudinal axis. Theydiffer from one another only in the electrode geometry. The width of thestrip-shaped electrodes 3, 4 as well as their distance from each otheris 3 mm and 4 mm respectively in the first case and 1 mm and 10 mmrespectively in the second case. In the first case (FIG. 2, above) theelectrodes 3, 4 are particularly easily identified. They stand out asdark regions from the wall of the cover 5, which exactly like theopposite wall of the base 6 appears bright due to the reflected andscattered fluorescent light of the glass. The length of the electrodesis 35 mm in each case. In both cases, but particularly evident in thesecond case (FIG. 4) it can be seen that the auto-luminescence of thedischarge is separated from the interior wall of the cover 5 by a darkzone 12 between the electrodes 3, 4. That is to say, that the discharge11 has a spacing from the surface of the interior wall in theaforementioned region. Viewed in the direction of the longitudinal axisof the discharge arrangement 1, the discharge 11 has a trough- orchannel-shaped appearance (in FIGS. 2 and 4 indiscernible due to thedirection of sight, compare FIGS. 1a and 1b).

If less power is coupled into the discharge arrangement,--for example,by reducing the voltage amplitude--the continuous, channel-shapeddischarge structure splits into individual structures that, as seen inFIG. 1a, also stand out from the dielectric surface. The individualstructures have a delta-shaped form (Δ) which widens in the direction ofthe (momentary) anode. In the case of alternating polarity of thevoltage pulses of a dual-sided dielectrically impeded discharge thereappears visually an overlap of two delta-shaped structures.

FIGS. 3 and 5 show respectively details from the temporalcharacteristics of voltage U(t) and current I(t) measured at theelectrodes during the operation in accordance with FIGS. 2 and 4,respectively. A comparison of both Figures substantiates the influenceof the electrode geometry on current and voltage outlined in theintroduction. In the following table the most important electricalparameters are compiled:

    ______________________________________                                        U.sub.p       T.sub.u                                                                              f.sub.u   w     P                                        ______________________________________                                        FIG. 3  -2.5 kV   1 μs                                                                              80 kHz  9.26 μJ                                                                          0.74 W                                 FIG. 5  -3.4 kV   1 μs                                                                              80 kHz  8.87 μJ                                                                          0.71 W                                 ______________________________________                                         Table: Measured values of electrical parameters of the two discharges         represented in FIGS. 2 and 4.                                            

In the Table, U_(p), T_(u), f_(u), w and P denote the height of thevoltage pulses (in reference to the voltage during the pause duration),the width of the voltage pulses (full width at half height), the pulserepetition frequency, the electrical energy per pulse and the timeaverage of the electrical power coupled in.

FIGS. 6a and 6b show the schematic representation of the cross-sectionand the top view (looking towards the base) of a lighting system 14designed for operation according to the invention. The lighting system14 consists of a flat discharge chamber 15 with a rectangular base andfive strip-shaped electrodes 16-20 as well as a voltage source 27, whichgenerates a sequence of voltage pulses during operation. The dischargechamber 15 itself consists of a rectangular base plate 21 and atrough-like cover 22. The base plate 21 and the cover 22 are connectedto one another in a gas-tight manner in the region of theircircumferencial edges and so enclose the gas filling of the dischargelamp 14. The gas filling is xenon at a pressure of 10 kPa. Theelectrodes 16-20 have equal width and are applied to the exterior wallof the base plate parallel to and equidistant from one another. This isimportant in order to ensure the same conditions for all dischargesbetween the respectively neighboring electrodes. As a result, when asuitable sequence of pulses is applied, an optimum radiant efficiencyand homogeneity of the luminance is achieved. For this the electrodes16-20 are alternately connected to the two poles 23, 24 of a voltagesource. That is to say, the electrode 16 and the two subsequent evennumbered electrodes 18 and 20 are connected to the first pole 23 of thevoltage source. In contrast the two odd numbered electrodes 17 and 19respectively are connected to the other pole of the voltage source.Sprayed onto the interior wall of the cover 22 and the base 21 is aphosphor coating is which converts the VUV (Vacuum Ultraviolet) and UV(Ultraviolet) radiation of the discharge 26--which is depicted here onlyin a rough schematic illustration--into (visible) light.

What is claimed is:
 1. Method for operating by means of dielectricallyimpeded discharge an incoherent emitting radiation source (1; 14),specifically a discharge lamp (14) having an at least partiallytransparent discharge chamber of electrically non-conductive materialwhich is sealed (2; 15) and filled with a gas filling or is open andthrough which a gas or gas mixture flows, and having electrodes (3, 4;16-20) which are separated from one another and from the interior of thedischarge chamber (2; 15) by dielectric material (5; 21), characterizedin that the electrodes are located next to one another in a common planeand on a common surface of said dielectric material and are connected inalternating fashion to the poles (23, 24) of a voltage source thatdelivers a sequence of voltage pulses which are separated by pauses, sothat a spatial discharge (11; 26) is generated in the interior of thedischarge chamber (2; 15) which has a spacing from the surface of theinterior wall of the discharge chamber in the regions between electrodesof different polarity (3, 4; 16, 17; 17, 18; 18, 19; 19, 20).
 2. Methodaccording to claim 1, characterized in that the pulse width lies in arange between 0.1 μs and 10 μs.
 3. Method according to claim 2,characterized in that the pulse width is in the range between 0.5 μs and5 μs.
 4. Method according to claim 1, characterized in that the pulserepetition frequency lies in the range between 1 kHz and 1 MHz. 5.Method according to claim 4, characterized in that the pulse repetitionfrequency lies in the range between 10 kHz and 100 kHz.
 6. Methodaccording to claim 1, characterized in that the voltage pulses have asemi-sinusoidal shape.
 7. Method according to claim 1, characterized inthat the pulse height lies in the range between about 100 V and 10 kV.8. Method according to claim 1, characterized in that the wall (5; 21)of the discharge chamber (2; 15) serves as dielectric between theelectrodes (3, 4; 16-20) and the discharge (11; 26).
 9. Method accordingto claim 8, characterized in that the electrodes consist of electricallyconductive strips (3, 4; 16-20) which are arranged next to one anotheron the exterior of the wall (5; 21).
 10. Method according to claim 9,characterized in that the number of the strips (16-20) is larger thantwo and the strips are arranged equidistantly on the exterior of thewall (21).
 11. Method according to claim 1, characterized in that theinterior surface of the wall (21) of the discharge chamber (15) isprovided at least partially with a phosphor coating (25).
 12. Lightingsystem with a radiation source, specifically a discharge lamp (14) witha voltage source (27) which supplies voltage to the radiation source,whereby the radiation emitted from the radiation source is incoherent,said radiation source (14) being suited for a dielectrically impededdischarge, having an at least partially transparent discharge chamber ofan electrically non-conductive material which is either sealed (15) andfilled with a gas filling or is open and through which a gas or gasmixture flows, and having electrodes (16-20) which are separated fromone another and from the interior of the discharge chamber (15) bydielectric material (21) and are connected to the voltage source (27),characterized in that the electrodes are located next to one another ina common plane and on a common surface of said dielectric material andare connected in alternating fashion to the poles (23, 24) of thevoltage source (27) which is capable of delivering a sequence of voltagepulses which are separated by pauses, so that a spatial discharge (26)is generated in the interior of the discharge chamber (15) which has aspacing from the surface of the interior wall of the discharge chamberin the regions between electrodes of different polarity (16, 17; 17, 18;18, 19; 19, 20).
 13. Method according to claim 2, characterized in thatthe wall (5; 21) of the discharge chamber (2; 15) serves as dielectricbetween the electrodes (3, 4; 16-20) and the discharge (11; 26). 14.Method according to claim 3, characterized in that the wall (5; 21) ofthe discharge chamber (2; 15) serves as dielectric between theelectrodes (3, 4; 16-20) and the discharge (11; 26).
 15. Methodaccording to claim 4, characterized in that the wall (5; 21) of thedischarge chamber (2; 15) serves as dielectric between the electrodes(3, 4; 16-20) and the discharge (11; 26).
 16. Method according to claim5, characterized in that the wall (5; 21) of the discharge chamber (2;15) serves as dielectric between the electrodes (3, 4; 16-20) and thedischarge (11; 26).
 17. Method according to claim 6, characterized inthat the wall (5; 21) of the discharge chamber (2; 15) serves asdielectric between the electrodes (3, 4; 16-20) and the discharge (11;26).
 18. Method according to claim 7, characterized in that the wall (5;21) of the discharge chamber (2; 15) serves as dielectric between theelectrodes (3, 4; 16-20) and the discharge (11; 26).